Communication systems take many forms. One type of communication system is a multiple access spread-spectrum system. In a spread-spectrum system, a modulation technique is utilized in which a transmitted signal is spread over a wide frequency band within the communication channel.
Three general types of spread-spectrum communication techniques exist, including direct sequence modulation, frequency and/or time hopping modulation, and chirp modulation. In direct sequence modulation, a carrier signal is modulated by a digital code sequence whose bit rate is much higher than the information signal bandwidth.
These direct sequence spread-spectrum communication systems can readily be designed as multiple access communication systems. For example, a spread-spectrum system may be designed as a direct sequence code division multiple access (DS-CDMA) system. In a DS-CDMA system, communication between two communication units is accomplished by spreading each transmitted signal over the frequency band of the communication channel with a unique user spreading code. As a result, transmitted signals are in the same frequency band of the communication channel and are separated only by unique user spreading codes. These unique user spreading codes preferably are orthogonal to one another such that the cross-correlation between the spreading codes is approximately zero.
It will be appreciated by those skilled in the art that several different spreading codes exist which can be used to separate data signals from one another in a DS-CDMA communication system. These spreading codes include but are not limited to pseudonoise (PN) codes and Walsh codes. A Walsh code corresponds to a single row or column of the Hadamard matrix.
Further it will be appreciated by those skilled in the art that spreading codes can be used to channel code data signals. The data signals are channel coded to improve performance of the communication system by enabling transmitted signals to better withstand the effects of various channel impairments, such as noise, fading, and jamming. Typically, channel coding reduces the probability of bit error, and/or reduces the required signal to noise ratio usually expressed as error bits per noise density (i.e., E.sub.b /N.sub.0 which is defined as the ratio of energy per information-bit to noise-spectral density), to recover the signal at the cost of expending more bandwidth than would otherwise be necessary to transmit the data signal. For example, Walsh code words can be used to channel code a data signal prior to modulation of the data signal for subsequent transmission. Similarly PN spreading codes can be used to channel code a data signal.
However, channel coding alone may not provide the required signal to noise ratio for some communication system designs which require the system to be able to handle a particular number of simultaneous communications (all having a minimum signal to noise ratio). This design constraint may be satisfied, in some instances, by designing the communication system to coherently detect transmitted signals rather than using non-coherent reception techniques. It will be appreciated by those skilled in the art that a coherent receiver requires less signal to noise ratio (in E.sub.b /N.sub.o) than that required by a non-coherent receiver having the same bit error rate (i.e., a particular design constraint denoting an acceptable interference level). Roughly speaking, there is a three decibel (dB) difference between them for the Rayleigh fading channel. The advantage of the coherent receiver is more significant when diversity reception is used, because there is no combining loss for an optimal coherent receiver while there is always a combining loss for a noncoherent receiver.
One such method for facilitating coherent detection of transmitted signals is to use a pilot signal. For example, in a cellular communication system the forward channel, or down-link, (i.e., from base station to mobile unit) may be coherently detected, if the base station transmits a pilot signal. Subsequently, all the mobile units use the pilot channel signal to estimate the channel phase and magnitude parameters. However, for the reverse channel, or up-link, (i.e., from mobile to base station), using such a common pilot signal is not feasible. As a result, those of ordinary skill in the art often assume that only non-coherent detection techniques are suitable for up-link communication.
A solution for the need for a coherent up-link channel is found in U.S. Pat. No. 5,329,547 to Fuyun Ling, commonly assigned together with this application to Motorola, Inc. This patent discloses the introduction of reference bits into the information datastream prior to spreading and transmission, and the subsequent extraction of these reference samples and their use in forming an estimate of the channel response. This estimated channel response is in turn used to coherently detect estimated data symbols.
While this solution allows for coherent detection, it assumes that more or less standard phase-locked loops (PLL's) are used for frequency offset estimation. However, such techniques do not fully exploit the known synch pattern.
Phase locked loops, or PLLs, are well known in the art. A PLL circuit is usually formed as a phase detector fed by input and feedback signals, a loop filter and a voltage controlled oscillator for producing a sine wave (i.e., the feedback signal). A basic PLL compares its estimated frequency, the sine wave, with the noisy input signal using a phase detector. An ideal phase detector followed by a loop filter will form a noisy estimate of the phase difference between the input and the VCO (voltage controlled oscillator) output. The VCO thus acts on the loop filter output to create the PLL estimate of a sinewave with the phase (and thus frequency) of the input.
While an elemental PLL is reasonably good at tracking phase for most applications, it is not as good at acquiring or tracking signals with large frequency errors. A PLL is characterized by a pull-in range B.sub.p. However, as B.sub.p increases, so does the variance of the phase error. AFC (automatic frequency control) units, FLLs (Frequency Lock Loops), or PLL's with phase and frequency detectors are often used to track such signals. These circuits typically produce an estimate of the average input frequency only, and additionally require an elemental PLL if the phase is to be acquired. However, in wireless communications AFC design has been constrained by circuit complexity, so system designs have typically made frequency accuracy constraints somewhat loose to avoid prohibitive costs in complexity or processing requirements.
However, with the introduction of more optimal modulation schemes such as QPSK (quaternary phase shift keying), more precise frequency estimates--within 30-60 Hz (Hertz)--are often needed. This is particularly true of applications such as coherent reception of DS-CDMA (direct sequence code division multiple access) spread spectrum signals, where the signal to noise ratio of the samples containing the frequency information is around 0 decibels (dB) (i.e., noise power equals signal power), and the frequency error may be .+-.1000 Hz or more before correction. These frequency errors may arise, for example, from the transmitter/receiver clock not being perfectly locked due to inaccuracies in the crystal oscillator, as well as from large Doppler frequency shifts (such as from vehicles moving at high speeds in open spaces). Coherent DS-CDMA systems such as that described in U.S. Pat. No. 5,329,547 and co-pending U.S. application "Method and Apparatus For Coherent Communication Reception in a Spread-Spectrum Communication System" by Ling et al., filed Feb. 28, 1994, and commonly assigned together with this application to Motorola, both of which are incorporated herein by reference, allow about 200 ms or less for initial acquisition and need the error after acquisition to be less than 100 Hz. However, at such wide frequency deviations in such short time periods, a typical AFC or PLL would not be able to lock on or track the signal being received with any reasonable degree of accuracy. There thus remains a need for an improved AFC/PLL which compensates for these and other problems.